The present invention relates in general to driver circuits and, more particularly, to current limiting a half-bridge driver for an airbag actuator.
Automobile manufactures regularly include airbags as standard equipment in their vehicles to comply with federal regulations. Sensors placed at strategic locations in the vehicle detect a sudden deceleration or acceleration incident to a collision and provide an electrical signal to the airbag actuator. Upon receiving the sensor signal indicating the sudden deceleration, the airbag actuator sends a firing signal to a squib, which is a detonation wire or device used to ignite an explosive charge. There is one squib physically attached to each airbag. The explosive charge releases a gas that inflates the airbag to protect the occupants of the vehicle during the impact.
The airbag and squib are physically located to protect the occupants, for example in the steering column, the passenger dashboard, door panels, etc. The sensors are typically located near the points of highest probability of impact generally around the perimeter of the vehicle, e.g. side doors, and front and rear bumpers. The actuator module may be centrally located, for example toward the rear of the engine compartment or behind the instrument panel. Electrical wiring harnesses interconnect the sensors, airbag actuator, and squibs.
The airbag actuator includes a high-side driver coupled between a high voltage power supply and one terminal of the squib, and a low-side driver coupled between a second terminal of the squib and a power supply conductor operating at ground potential. Upon receiving the sensor signal, the high-side driver and low-side driver conduct a 2 amp firing current through the squib, which is sufficient to fire the explosive charge. The airbag actuator has multiple sets of high-side drivers and low-side drivers, one set for each squib and airbag combination. For example, a first pair of high-side/low-side drivers operate the squib and airbag for the steering wheel, a second pair of high-side/low-side drivers operate the driver door panel, and so on.
The power supply must provide 2 amps of current for each of the multiple squibs, possibly simultaneously. For a vehicle with six airbags, the power supply must be capable of sourcing 12 amps of current. The power supply is typically implemented as a large bank of capacitors that are continuously charged to provide the necessary current even in the event of a primary power failure in the vehicle, which is possible during a collision.
The high-side driver and low-side driver are further current limited so that in the event of an electrical short in the wiring harness no single driver pair can draw excessive current from the power supply. Even in the event of a fault in one driver pair, the power supply must have current source capacity for the remaining operative driver pairs. The low-side driver is typically current limited to 2 amps. It is difficult to current limit the high-side driver to the same value as the low-side driver because of the potential for unstable operation and oscillations in the driver control circuits. Consequently, the high-side driver is current limited to about 2.5 amps. Unfortunately, the higher current limit on the high-side driver increases the current source requirement of the capacitor bank and increases the peak current carrying capacity of the high-side driver conduction path to 2.5 amps. The power supply and high-side driver must be made physically larger to handle the larger currents. The higher current carrying capacity of the high-side driver also decreases the robustness of the airbag actuator.
Hence, a need exists to current limit the high-side driver and low-side driver without increasing the capacity of the capacitor bank or driver conduction paths.